Frequency measuring system



E. s. CLAMMER ET AL FREQUENCY MEASURING SYSTEM Filed may 27, 1946 Oct.12, 1948.

Patented Oct. 12, 1948 FREQUENCY MEASURING- SYSTEM Edward S. Clammer,Merchantville, and Milton J. Ackerman, Camden, N. J., and George A.Barton, Bridgeport, Conn., assignors to Radio Corporation of America, acorporation of Delaware Application May 27, 1946, Serial No. 672,440

(Cl. Z50-39) 11 Claims.

This invention relates to frequency measuring systems for determiningthe frequency of unknown radio frequency signals, and is particularlyadapted for determining the frequency of radio frequency signals inultra high frequency and microwave ranges.

Wave meters, employing variably tuned circuits, are commonlyused inmeasuring unknown signal frequencies. For measurements in the ultra highand microwave ranges tuned wavemeter circuits may take the form ofvariableconcentric lines. Such meters are accurate only within certainlimits and have the disadvantage that an appreciable amount of energyfor their operation is required from the signal supply source beingmeasured, thus precluding measurement accuracy where the signal energymay be relatively low, as in the ultra high and microwave frequencyranges.

It is the primary object of this invention, therefore, to provide animproved frequency measuring system which avoids the use of tunedcircuits responsive to the signal frequency to be measured, therebypreventing energy losses incident to the use of such circuits, and whichat the same time may be calibrated with reference to a crystalcontrolled or other fixed frequency source.

More particularly, it is an object of this invention, to determine thefrequency of unknown high frequency signals, extending through a range,for example of from 1 mc. to- 10,000 mc. and higher, without employingtuned measurement circuits responsive thereto, and without otherwisewithdrawing any appreciable energy from the frequency source beingmeasured, whereby greater measurement accuracy is attained at allfrequencies and effective frequency measurement is made possible at highfrequencies of the order above referred to.

It is also an object of this invention, to provide an improved frequencymeasuring system which may operate .in a relatively. low frequency rangecorresponding to the modulation width or deviation frequency, and usefuldeviation frequency harmonics, of a frequency modulated oscillator, thefundamental frequency or a harmonic of which is caused to beat withsignals or an unknown higher frequency, toiprovide ameasure of the'frequency of said unknown higher frequency signals.

More specifically, it is an` object of this invention, to provide afrequency` measuring system having means for deriving `and determining aharmonic of the deviation or modulation frequency of a lowerfrequency-modulated signal as a measurement index of the frequency ofsaid signal and of an equal or higher frequency unknown signal whenbeating ytherewith to zero beat, whereby tuned measurement or wave metercircuits and the like, such as variable concentric line circuits,responsive to the unknown signal frequency, and energy losses inherentin the use thereof, may be avoided.

It is also a further object of theinvention, to provide a measuringsystem for radio frequency signals wherein, the frequency deviation of afrequency modulated signal, vand the harmonics ofsaid'frequencydevlation are utilized as a measure of the frequency of`an unknown signal, and such frequency deviation andthe useful harmonicsthereof are arranged to be measured on the cathode ray pattern vof asimple oscilloscope, thereby eliminating the use of frequency responsivemeters and the like, and associated circuits.

The invention will, however, be better Iunderstood from the followingdescription when considered in connection with lthe accompanyingdrawingand its scope will be pointed out in the appended claims.

in thedrawing,` Figures l and 2 are frequency graphs representing aprinciple of operation of the invention, and Figure 3 is a schematiccircuit diagramV of a frequency measuring system embodying theinvention.

' Referring to Fig. l, the horizontal abscissa line E represents afrequencyhncreasing in a positive directionto the right' from amid-frequency line 6, -for a frequency modulated signal the energyoutput content of which is indicated by the vertical height ofl theline.Assuming such ay signal to' be frequency modulated with a deviationfrequency of i kc.,` as indicated, the energy envelopeof the signal isindicated by the restan gular boundary lines l, 8 and 9.

IfA nowas indicated the Fig. 2, a higher harmonic `I0 ofthe frequencymodulated signal 6 of Fig. 1 is'derived, it will be found that thefrequencymodulation envelope Il, l2, v13 will have increased in width toprovide a resultant deviation frequency of l1000 kc. in the case of thetenth harmonic, for example. Therefore, by measuring the deviationfrequency ofthe signal l0, and knowing its original vdeviation or sweepfrequency, the lharmonic order of vthe signal may readily be determinedwithout resorting to measurement of frequencies higher than the devia*tion frequency harmonic, or 10,00v kc. in the present, example. Assuming-that 'the ,original signal 3 was 495 mc., the 10th harmonic will have,as indicated in Fig. 2, a, frequency of 4950 mc.

In accordance with the invention, the frequency of the signal 6 isadjusted until a harmonic such as I is at zero beat with an unknownsignal, thereby eliminating the harmonic Il) and leaving only theharmonic of the limit or deviation frequency of the signal 6, which isindicated between limits I2 and I3 of Fig. 2. If this is meas.- ured interms of multiples of the swing or deviation frequency of the signal 6,the harmonic order of the deviation frequency of the signal I0 and theharmonic order of the signal III with respect to the signal 6 isdetermined. Therefore the unknown signal frequency may be determined byreference to the frequency 6 multiplied by the harmonic order justdetermined.

Thus it will be seen that such frequency determination obviates takingany actual frequency measurement at the frequency of the harmonic I0 orof the unknown signal. It will be appreciated, therefore, that inmeasuring or determining the frequency Yof ultra high frequency signals,this system has the advantage that the actual frequency determination ismade at relatively low frequencies. This principle is particularlyuseful in measuringy ultra high frequencies and microwave frequencies ofthe order of 300 me. to 60,000 mc. and higher, since all measurementsand computations are made with respect to the deviation frequency of aknown frequency modulated signal and the harmonics thereof.

A system for carrying out the above measurement principles, and arrangedfor making frequency measurements in a range between 100 mc. and 10,000mc'., is shown, by way of example, in Fig. 3, to which attention is nowdirected.

In Fig. 3, yI5 is a frequency modulated oscillator having apredetermined known frequency deviation and tunable in a range betweenthe lowest frequency to lbe measured, or slightly below 95 mc. in thepresent example, and a higlher frequency such as 500 mc. in the presentexample, having useful or strong higher harmonics up to the highestfrequency to be measured. This will provide 60,000 mc. for the 120thharmonic and, in the present example to be considered, 10,000 mc., forthe th harmonic. It is preferable also that the tuning range of thefrequency modulated oscillator be at least twol to one.

The oscillator I5 comprises an oscillator tube h6 having a tuned tankcircuit I1 to which is connected suitable modulating means, such asvariaprovide uniform frequency swing above and below the centerfrequency or either above or below to any desired degree. In the presentexample, the frequency swing may be assumed to be symmetrical about thecenter frequency.

In addition, a compensating device I4 is provided in circuit with themotor means 2I and con nected as indicated, to be actuated by operationof the tuning dial 20 to maintain the adjusted deviation frequencyconstant as the tuning of the oscillator is varied.

The output of the oscillator is supplied to a pickup loop 26 at one endof a concentric trans- Y mission line 21, the opposite end of which isconnected or plugged into the output terminal 28 of a signal supplysource, indicated at 29, for which an exact frequency measurement isdesired or the frequency of which isunknown.

The signal supply source is preferably unmodulated for measurement, andprovides a frequency which is equal to or higher than the lowest tunablefrequency of" the oscillator I5 which has been assumed above to extendbetween 95 mc. and 500k mc. Also for the purposes of the presentexample, the frequency of the signal supply source A29 may be assumed tofall Within the range of 95 mc. to

ble modulating capacitor I8, and variable tuning means, such as avariable tuning capacitor I9, the latter being connected, as indicated,with a calibrated frequency indicating or tuning dial 20, by iwhich theoscillator is variably tuned. It is preferable that the tuningadjustment of the dial or other indicating means be readable to a fairdegree of accuracy, such as i5 per cent.

The modulatingmeans I8 is connected with motor means 2 I energized fromalternating current supply leads 22 through a potentiometer device 23and a control switch 24. The potentiometer device is arranged as a sweepor deviation frequency control means having a calibrated dial 25 onwhich adjustments may be made for varying the oscillatory movement ofthe motor means 2 I, through a greater or less range of travel.

This represents any suitable arrangement for modulating the oscillatorat the rate of the alternating current supplied, to any predetermineddeviation frequency, such as 100 kc'., as indicated on the dial. Themotor means may be operated to The transmission line 21 is tapped toprovide a connection through a line 30 with an R.F. signal mixer device3i, preferably of the crystal rectier type, as indicated, and capable ofoperating in a frequency range between the lowest frequency of theoscillator I5 and the highest frequency of the unknown source 29.

It will be seen that, with this arrangement, harmonic signals from theoscillator I5 and a signal to be measured, from the source 29, areapplied to the mixer means 3l and the difference frequency outputtherefrom is conducted from the mixer output circuit 35, through a wideband second mixer represented by the amplifier tube 36, and thencethrough a band pass amplifier represented by theramplier tube 31, to thevertical deflection terminal 38 of an oscilloscope 39, a suitable out-`put coupling capacitor 40 `and a transmission line connection 4I beingprovided between the amplifier 31 and the oscilloscope.

Ilhe input circuit of the mixer stage 36 includes an input grid 45 and agrid-cathode return circuit in which is located a high frequency filter46, effectively across the output circuit 35` of the R.F. mixer 3|, forremoving the high frequency signal component therefrom. The cathodecircuit of the mixer 36 includes a resistor or impedance 41, to whichmay be connected, through a switch I8 and a transmission line 49, avariable frequency or interpolation oscillator represented at 5I),having an accurately calibrated tuning means represented by tlhe dial5I. In the present example this may be considered to be tunable between10 mc. and 20 mc. Accordingly, the second mixer is arranged to beresponsive to a low pass band from 10 C. P. S. to 20 mc. thereby toinclude the highest frequency of the oscillaputcircuit 52 through anR.F. choke coil 51 and a switch 58, and applied to an audio frequencyamplifier: 59,1' theoutput circuit 60..of

Whichi :is;.aconnected. with. .a pair. .of headphonesv 6|, for purposesWhic-hfwill. hereinafterappear..

The1.amplifier-59 ioffthepresent. example' is preferablyfresponsivewto afrequency range :of 10. C. Pff S. '1:01.15'Y kc..

The-band pass amplifierl` is. preferably madev responsive :in a ifrequency range extending .downY to vtheilowestaideviation, frequency.whichfmay :be

applied: fto. they oscillator 15,:A which.' in: thisl case is..80ike.;and1having..anupperflimitxof .frequency ample, to. i5 peracentuagreaterdegree: :ofvaecue:P

racy may. be obtained. with the; system :showmfor i' determining the.unknownifrequency through they. use. -of externalAcalibrationzarneans'.. readings accurateto .i-.0051per ,centitrnay 'beob-:.- f

`lewtained with;:the present .measurementssystem.:l

suiicient tow pass;.the. highest. i desired harmonic.

of '-.the .original deviationnfrequency.-` Inathis. case,.

for measurements of the. order of .10,0001mc.; calling..:fo'r;the-20th2iharrnoniccy of` the N:highest freqquency (50.0: i me.)Jestablished i for..V .thergoscillaton :f |51'V the! upper limiti willbe.;2 11net.; Fonga Lrange).` ofuGgOOO me.,l calling v.forrtlie120thfharmonidorr: example, 'the upper limitzofthe range; oftheiband f;

passffamplier wouldtbe kapproximately 12;mc.;

assuming an originali.; deviation.. frequency ofv 1' Theihighestzfrequency to be :passed .by the` wide; :eband.'mixerz;36i,'fthe.:band .passamplifier 31 iandai the frequencyindicating device 39 may-be. 'de-.1`

rivedrfrom :the formula:

(1l) Freq...(max.),=

Highest freq. of oso. l5 `f deviationy freq.` .of iosc.. l5

In the `present example, this wouldbe determined as Highest freq. ofsig.y sourceZQ' v,

Itis necessary to-provide, in connectionwwithw the circuitbetweenv theoscilloscope=and the- 1 RI'eFf/mixer'ii l, -and 'preferably inconnection xwith the lband fpass amplifier circuit; means for nicas-'- luring the-highestfrequency of thee-deviation frequency'harmonic 4whichis-pass-ing through 1the amplifier system' and appearing conVvtheoscillof Oscillatorcrcuits forthis' purposeare undesirablebecause/of` `-the introduction vofharmonics whichm-ay-provide--falsereadings.- In the-pres-4 ent system, a 1tunable -absorptionmarker fortheoscilloscope patternl is provided to produce-fa null I in theIpattern which is -movabl'e'along -the pat--V territo-the endthereof fordetermi-ning the ex-'Y- tremev frequency ranger -of .they circuit.Y

be-.connected in-y circuiti with thecapacitor l6 5l by suitablemeans,such as a vselectoriswitchHIL-'- The absorption marker circuit isconnected-l across-the input. impedance1.56..10f.I the bandpassamplifier 315; througha switch .-12. and a connectinguf liner-1321Further discussion .of the .calibrationof thewdia'lmillappearlhereinafterl AThe..tuning range offtheabsorption. marker ispreferably the same as that ofizthe band pass -arnplier;.namely;=..

fkc.; to. 2 me. inl Atheupresent. example.

Thechorizonta'l deflection z. terminali, 15. is.: con-g.

necteduwith aa .suitable-phasing. .control-.1:circ-uit.16;. hailing-aVariablephasing. control element. "IT: and

isgsuppliedz with suitable alternating current-4 throughysupply fleadsindicated' f at i .1 8.3i; a icontrol@` supply f requencygis Inecessarily the.v same pas pizo-l I vided iat t 22; for .themodulation.y rate. -of oscillator V vand sinceltheband passamplifier-311. prevents theYN whenthe Ioscillator isf ffurthercalibrated'.

A prefer-redea-libration meansifor the oscilla-torn i5 is crystalcontrolled.forgreater-accuracy,andt stability, .f and-mayvcomprise afixed ifrequency -oscill'ator 85 '-1 having .zzcrystalifrequency:control?i means indicatedlat 8 6,-'zand' a .harmonievfrequency.;v amplifier'.y indicatedi at` 81:', s' providingthe..afunda;+i-

mental and harmonie frequcnciesfothee osci-llato1-\..N

at an output terminallindicatedatlzto which l:

the .terminal 28 `Lofithe signal supply zmeans, f as; f indicated. in`dol'ited lines. In .the present-.exi

thereby providinga fundamentalof 10.fmc.; andl a :series ofY harmonicsspaced 10 mc.: apart. n The interpolation oscillator 5U, tunable:,.betweenz 10 and 20.1nc., isused.iniconjunctioncwith the oscila lator35 for Calibrating theoscillator.I5,.:as.-Mfill f l1creinafterv appear.r

The. -.operation is as; follows As previously pointed out,U theoscillator. 'l5 .Eis an .ultra-z-,high-f: frequency oscillator, Variablytunable between 951 nic.l and .500.: mc.. and 'frequency modulated at= a.i GO-Lcyclez rate, for11eXample,-.with a 4predetermined-': deviation orsweep .frequency. such.7 ase- P kc.; which .is .accurately .determined'and adjusteduin connectionv with. 1 the` :calibrated i sweep :control23;.-25 and .the compensation .controLmeans M. i.

The. medulated :output-i signalV of. the: oscillator is mixed-inthecrystalfmixer :3l :.with1an1R1-F: signal the. frequency Aof which ais'.to lbe.=rneas1n'ed y i and. .preferablyxwithout modulation, r introducedat :the input terminal 28'1'froma ,signalsource 29. The generatororioscillator: I 5 is vVariably tuned until a harmonic of-.thesignalxtherefrom produces Y a zero beat iwith the unknownzsignalffromthcsource 29. 'Ihiszero-beatf-appears in -.the.0scillo. scope pattern,indicatedfatyasa null 915.-. The. bandof frequencies'representedv inthe.oscillor-f :scope pattern :extend fromz'zero tosome-frequencydeterminedby '-.the-orderi'of thevharmonic offthez: n

passage of: frequencies below -80 .kc., the'null 9i..a .appears whenzthetwo signalsare at zer-.o beat,- -as the lower. output frequenciesdof-not,- therefore,.- reach v:the oscilloscope. In: addition toatheffilter...

...45g the upper'cut-off frequency of .the band mass.-

Frequency 36 and 3l pass the modulation frequency and its harmonics upto any desired harmonic, which in the present example is the 20th,although for practical purposes, as hereinbefore mentioned, the 120th`harmonic may be passed if desired for measurement of higher frequencyunknown signals. In this connection, it may be pointed out that for themeasurement of higher frequency signals, the tuning range of theoscillator I may be increased, for example, to a range between 1000 Inc.and 3000 mc. For lower frequencies, the range may be lowered, forexample, to cover a tuning range of 1 to 5 mcs.

The band width of the oscilloscope pattern between the limits 92 and 93may now be read by Vthe absorption marker, which is connected in circuitby closing the switch 'i2 to absorb some of the energy of the harmoni-csignal passing through the amplifier 3'! to the oscilloscope.

In the present example, the absorption marker is connected across theinput resistor of the amplifier 3l as an impedance whose value isminimum at resonance. The tuning dial 59 is tuned until a slight null 95appears in the oscilloscope pattern. This indicates the frequency towhich the marker is tuned. By operating the phasing control 73, the zerobeat null 9| may be made to appear at one edge 92 of the pattern asshown and the highest sweep or deviation frequency would then appear onthe opposite edge 93.V By turning the dial 66 to move the marker null tocoincide with said opposite edge 93 of the oscilloscope pattern, thefrequency reading of the calibrated dial of the marker will thenindicate the limit frequency in the pattern.

Since the deviation frequency of the oscillator l5 is known (i100 kc. inthe present example), the dial reading, in frequency, of the absorptionmarker may be divided by that value to determine the order of theharmonic of the original signal at the oscillator i5 which is then atzero beat with the unknown signal from the source 29. Therefore thisresult is .an index of the frequency of the source 29. The latterfrequency may then be obtained approximately by multiplying thefrequency reading on the dial 20 of the oscillator I5 by the harmonicorder derived as above. Assuml ing that the dial 65 of the absorptionmarker then reads 1000 kc., a harmonic order of I0 is indicated, and ifthe dial 20 of the oscillator I5 then reads 495 mc., the unknownfrequency is 10 times 495 mc. or 4950 mc.

It is obvious, therefore, that the calibrated tuning dial 66 of theabsorption marker may be calibrated Yto read the multiplication factoror harmonic order instead of frequency, and may be reset to indicate thefactor directly, as, for example, in the position shown for the 10thharmonic. This simplifies the operation of measuring the frequency ofthe unknown signal, since it is then only necessary to multiply the dialreading at 65 by the dial reading at 29 to obtain a close approximationof the unknown frequency when the null 95 on the oscilloscope pattern isat the end of the pattern and the signal from the Aoscillator' l5 andthe unknown signal are beating at Zero beat.

Thus it will be seen that the harmonic of the deviation frequency is anindex of the frequency of the modulated signal and of the unknown signalwhen at Zero beat with the modulated signal. Furthermore, nomeasurements are required by this system which involve the tuning ofultra high frequency or microwave circuits or the absorption of anyappreciable energy at the frequency to be measured. Further inaccordance with the invention, exact frequency measurements to' 1.005per cent., for example, may then be determined if desired ashereinbefore referred to.

This entails (1) removing the frequency modulation from the oscillatorI5 by opening the switch 24 while maintaining the tuning of theAoscillator I5 for an exact zero beat of one of its harmonicsV with theincoming signal from thesource 29. The exact zero beat is preferablydetermined by audible means, such as the headphones 6l, by closing theswitch 58 and energizing the amplifier 59, and then observing theaudible Zero beat between the signal from the source 29 and from theoscillator I5 at the output of the wide band second mixer 36.

The next step (2) is to remove the concentric line 2l from connectionwith the terminal 28 and to reconnect it to the terminal 88 of theharmonic frequency amplifier 81 which in turn is connected with thefixed frequency oscillator 85. With the oscillator and harmonicamplifier energized, a.

harmonic of the 10 mc. signal will beat in the R.-F. mixer 3I with thesignal from the oscillator I5 to produce a difference frequency between10 and 2.0 mcs. at the output circuit 35 of Y the mixer 3 I.

For example, if the dial 20 of the oscillator I5 at the output circuit35 of the R.-F. mixer 3 I, and

all will therefore pass through the widek band mixer 36 which isarranged to pass frequencies up to 20 mc. YIt is obvious that other beatfrequencies will be attenuated.

The next step (3) is to close the switch 48 and place the interpolationoscillator 59 in operation to supply signals to the mixer 36. The dial5| is then adjusted until the interpolation oscillator provides a zerobeat with one of the above beat frequencies. Y headphones BI, the dial5l is read as accurately as possible and the reading is added to orsubtracted from the harmonic of the oscillator85 which isbeating withthe oscillator I5.

For example, if the dial of the oscillator I5 .reads 495+ mc. and thedial of the interpolation oscillator 50 reads 15,123 mc., it may bedetermined which multiple of 10 mc. plus 15.123 mc. equals approximately495-1- mc. The result is that the 48th harmonic or multiple of the 10mc.

output of the fixed frequency oscillator B5 satis-` fied thisrequirement. Therefore, the frequency of the oscillator I5 is 48x10 mc.or 480 mc., plus the dial reading 5Ik of the oscillator 50, or -|-15.123mc., which equals 495.123 mc. Since the calibrated tuning dial of theabsorption marker vindicated a harmonic order of 10, the frequency ofthe unknown source at 29 is 10 495.123 mc., or 4951.23 mc.

Frequency measurementsfor other frequencies are made in a similarmanner. The ultimate accuracy is largelyrdetermin'ed by the accuracywith Y This zero beat is noted in theV scribed in connection with acircuit for the measurement of present normal ultra high frequencysignals, it should be understood that it is not limited thereto and maybe applied in a similar manner to the measurement of any R.F. signal.

We claim as our invention:

1. In a frequency measuring system, a tunable frequency modulatedoscillator having a predetermined deviation frequency, means for mixingthe signal output of said oscillator with a second signal to be measuredto provide a beat frequency signal, means for varying the tuning of saidoscillator to provide zero beat at said mixer means between a harmonicfrequency signal therefrom and said second signal, whereby said harmonicfrequency signal equals the frequency of the second signal and providesa harmonic of said predetermined deviation frequency of a correspondingorder, tuning indicator means for determining the frequency of theoscillator when said harmonic frequency signal therefrom is at zero beatwith said second signal, and calibrated tuning and indicating meanscoupled to said mixer means and being selectively responsive to thedeviation frequency and harmonics thereof for determining the harmonicorder of said deviation frequency harmonic as an index of the frequencyof said signal and of the second signal to be measured for multiplyingwith said determined frequency of the oscillator to derive the frequencyof the second signal.

2. In a frequency measuring system, a tunable frequency modulatedoscillator having a predetermined deviation frequency, calibratedfrequency indicating means for said oscillator, means for mixing thesignal output of said oscillator with a second signal to be measured toprovide a beat frequency signal, means for varying the tuning of saidoscillator to provide Zero beat at said mixer means between a hormonicfrequency signal therefrom and said second signal, whereby said harmonicfrequency signal equals the frequency of the second signal and providesa harmonic of said predetermined deviation frequency of a correspondingorder, and calibrated tuning and indicating means coupled to said mixermeans and being selectively responsive tothe deviation frequency andharmonics thereof for determining the harmonic order of said deviationfrequency harmonic as an index of the frequency of said harmonicfrequency signal and of the second signal to be measured, and meansincluding a fixed frequency signal source for determining with apredetermined degree of accuracy, the frequency of said oscillator withreference to said calibrated frequency indicating means, thereby toprovide a multiplying factor for said index further to determine thefrequency of said second signal with a corresponding degree of accuracy.

3. A frequency measuring system comprising, in combination, means forproducing a frequency modulated signal having a predetermined deviationfrequency, means for mixing said modulated signal and a second signal tobe measured in frequency, to produce a beat frequency output, means forvarying the frequency of the first named signal to provide zero beatbetween a harmonic thereof and the second signal, band pass circuit forderiving the corresponding deviation frequency harmonic of said firstnamed signal from the beat frequency output of said mixing means, andmeans for determining the order of said last named harmonic as an indexof the frequency of the second signal, said last named means. includinga circuit coupled to the band pass circuit and tunable to said deviationfrequency harmonic, and a device couple-d to the output of the band passcircuit for indicating the tuning response of said last named circuit atsaid deviation frequency harmonic.

4. A frequency measuring system comprising, in combination, means forproducing a frequency modulated signal having a predetermined deviationfrequency, means for mixing said modulated signal and a second signal toproduce a beat frequency output, calibrated tuning means for varying thefrequency of the rst named signal to provide .zero beat between aharmonic thereof and the second signal, a band pass circuit for derivingthe corresponding deviation frequency harmonic of said first namedsignal from the beat frequency output of said mixing means, means fordetermining the order of said last named harmonic frequency as amultiplying factor for the frequency of the rst named signal toapproximately determine the frequencyv `of the second signal, said lastnamed means including a circuit coupled to the band pass circuit andtunable to said deviation frequency harmonic, and -a device couple-d tothe output of the band pass circuit for indi-eating the tuning responseof said last named circuit at said deviation frequency harmonic, andcrystal controlled means for further calibrating said first named meansto nally determine the frequency of the second signal.

5. A frequency measuring system comprising in combination, a frequencymodulated oscillator having a predetermined deviation frequency, signalmixer means coupled to said oscillator for receiving the signal outputtherefrom, means for applying a signal to be measured to said mixermeans, means for tuning said oscillator to provide zero beat at saidmixer means between a harmonic frequency signal therefrom and theapplied signal, calibrated means for indicating the frequency of saidoscillator as a first index of the frequency of said applied signal,band-pass amplifier means cou-pled to the output of said signal mixerhaving .a pass band which includes the deviation frequency of saidoscillator and a predetermined number of harmonies thereof, and meanscomprising a tunable circuit coupled to the mixer means and an indicatordevice -coupled to the band pass amplifier means for measuring theydeviation frequency harmonic at the output of the mixer device whensaid oscillator is tuned to provide said zero beat, as a second index ofthe frequency of sai-d applied signal.

6. In a frequency measuring system, a tunable frequency modulatedoscillator having a predetermined deviation frequency, calibratedfrequency indicating means for said oscillator, means for mixing thesignal output of said oscillator with a second signal to be measured toprovide a beat frequency signal, means for varying the tuning of saidoscillator to provide zero beat at said mixer means between a harmonicfrequency signal therefrom and said second signal, means for determiningthe harmonic order ofthe deviation frequency output of said mixer means,said last named means comprising an oscilloscope having a deflectionelement thereof coupled to said mixing means, a band-pass amplifier insaid coupling having a predetermined band-pass range including thefundamentalof said deviation frequency and a predetermined higherharmonic thereof, and a calibrated absorption marker for saidoscilloscope coupled to said amplifier and having a tuning range of theorder of the pass band of the amplifier.

7. A frequency measuring system comprising in combination, a frequencymodulated oscillator having apredetermined frequency deviation, acalibrated frequency indicating tuning device for said oscillator, atransmission line coupled to the oscillator for receiving a highfrequency signal to be measured in frequency, an R.F. signal mixerdevice connected With said transmission line responsive to saidoscillator and high frequency signals to provide both a beat frequencytunable to Zero beat by said oscillator and a resultant harmonic of theoscillator deviation frequency, a band pass circuit cou-pled to theoutput of the mixer ydevice providing a frequency pass lband for saiddeviation frequency harmonic, a tunable circuit coupled to the output ofthe signal mixer device and an oscilloscope device having a verticaldeflection control terminal connected With the output of the band passcircuit responsive to the deviation frequency harmonic at the output ofthe mixer device, said oscilloscope device having a horizontaldeflection control terminal, an alternating current modulation supplycircuit for the oscillator connected With said last named terminal tosupply deflection potential thereto, a phasing control network in saidconnection, and a calibrated frequency indicating tuning device for saidlast named tunable circuit providing a frequency multiplying factor forthe frequency indication of the iirst named calibrated frequencyindicating tuning device, as a measure of the frequency of said highfrequency signal.

8. In a frequency measuring system, a tunable frequency modulatedoscillator having a predetermined deviation frequency, means for mixingthe signal output of said oscillator with a second signal to be measuredin frequency, means for varying the tuning of said oscillator to provideZero beat between a harmonic frequency signal therefrom and said secondsignal,

calibrated frequency indicating means for sai-d',

oscillator, means for determining the harmonic order of the deviationfrequency of said harmonic frequency signal, said last named meanscomprising an oscilloscope having a vertical deflection element thereofcoupled to said mixing means, band-pass amplifier means in said cou`pllng having a predetermined band-pass range including the fundamentalof said ydeviation frequency and a predetermined higher harmonicthereof, a tunable marker circuit for said oscil-.QS5

loscope coupled to said amplifier and having a calibrated tuning rangeof the order of the pass band of the ampliermeans, and means forcalibrating said first named oscillator to a predetermined degree ofaccuracy to finally `determine the frequency of said signal to bemeasured, said2A last named means comprising a harmonic amplifier havingan output connection With said mixing means, a crystal controlledoscillator coupled to said harmonic amplifierV and tuned to a,

frequency providing a series of harmonics Within theY tuning range of.said iirst named oscillator, and a variably tunable interpolationoscilla- Ator Vcoupled to the input side of said amplifier Y fordetermining a harmonic of the crystal conbination of a frequencymodulated oscillator tunable in a range between the lowest frequency toeffective harmonicV of the order of the-highest V`frequency to bemeasured,V means providing `a calibrated tuning dialfor said oscillator,means for adjusting rthe frequency swing of said oscillator to provide apredetermined known deviation frequency therefor, a radio frequencysignal mixer device coupled to said oscillator to receive the signaloutput therefrom, said mixer device being ofthe crystal type, meansproviding a signal supply connection with said mixer device for applyingthereto signalsV to be measured in frequency, a second mixer meanscoupled to said radio frequency mixer device to receive the beatfrequency output therefrom, ay crystal controlled oscillator tuned to afrequency providing a series of harmonicswvithin the tuning range ofsaid rst named oscillator, a harmonic frequency amplifier coupled tosaid crystal'controlled oscillator to receive the signal outputtherefrom, means for connecting said radiol frequency mixer device withthe output of said harmonic amplifier, band-pass amplifier means coupledto the output of said second mixer device having a pass band whichincludes the deviation frequency of said frequency modulated oscillatorand apredetermined number of harmonics thereof," an oscilloscope devicehaving a viewing screen and khaving a Vertical deflection elementthereof coupled to theoutput of said band-pass ampli'- i-ler means,phasing means for said oscilloscope device, and means for measuring thehighest frequency of a deviation frequency band of signals applied tosaid oscilloscope through said bandpass amplifier, comprising a tunablesignal absorption circuit coupled toY said band-pass ampliiierwhereby itis effective rat its resonant fre- `line responsive to said oscillatorand Ahigh frequency signals to provide both a beat frequency tunable toZero beat by' said oscillator and a resulting harmonic ofthe oscillatordeviation frequency, a band pass circuit coupledto the out- Aput ofVthe'mixer device providing a frequency pass band for the deviationfrequency and the highest desired harmonic thereof, means comprisingvatunable circuit'coupled to the output of the signal mixer device and avfrequency responsive indicator device coupled to the output of the bandpasscircuit for measuringthe deviation frequency harmonic at theoutputof the mixer device when said oscillator is at zero beat With the highfrequency signal, thereby to provide a frequency multiplying factor forthe frequency indication of the first named-tuning device, as a measureofthe frequency of said-high frequency signal.

11. A frequency measuringrsystem comprisingY in combination, a frequencymodulated oscillator, a modulator device 4for applying a prede-fVtermined frequency deviation thereto, avcali- Ybrated frequencyindicating tuning device forV said oscillator, an alternating currentsupply circuit connected Withsaid modulator to provide a predeterminedmodulation rate for `the oscillator, a control circuit in said lastnamed connection having a movable control element responsive tooperation of the tuning device for maintaining the deviation frequencysubstantially constant as the oscillator tuning is varied, atransmission line coupled to the oscillator for receiving a highfrequency signal to be measured in frequency, an R.F. signal mixerdevice connected with said transmission line responsive to saidoscillator and high frequency signals to provide both a beat frequencytunable to zero beat by said oscillator arid a resultant harmonic of theoscillator deviation frequency, a band pass circuit coupled to saidmixer device for passing said deviation frequency harmonic, meanscomprising a tunable circuit coupled to the mixer device and a frequencyresponsive indicator device coupled to the band pass circuit formeasuring the deviation frequency harmonic at the output of the mixerdevice when said oscillator is at zero beat with the high frequencysignal, thereby to provide a multiplying factor for the frequencyindication of the rst named tuning device, as a measure' of thefrequency of said high frequency signal.

EDWARD S. CLAMMEIR.

MILTON J. ACIERMAN.

GEORGE A. BARTON.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,321,315 Peterson June 8, 19432,324,077 Goodale July 13, 1943

